The Shared Psychotic Disorder, also known as "folie à deux", is a mental disorder characterized by the development of psychiatric symptoms (psychotic symptoms) in an individual within the context of a close interpersonal relationship with another individual who has an established mental illness, usually with the presence of delusions. It is not considered a common disorder, its diagnosis is rare and it is a condition in which psychosocial factors are vital for its development.To present the case of two sisters, the social consequences and difficulties in the intervention as well as ethical issues related to involuntary hospitalization. To perform a brief review of the topic so as to have a framework for the case.Case report.This case makes evident the difficulty to achieve an adequate pharmacological and psychotherapeutic intervention because delusion symptoms can affect not only one but several family members. Furthermore, this case accounts both for individual risks inherent to the disease and for social risks involving third parties.